As compared with widely used amorphous silicon (a-Si), amorphous (non-crystalline), oxide semiconductors have high carrier mobility (also called as field-effect mobility, which may hereinafter be referred to simply as “mobility”), a wide optical band gap, and film formability at low temperatures, and therefore, have highly been expected to be applied for next generation displays, which are required to have large sizes, high resolution, and high-speed drives; resin substrates having low heat resistance; and others (see Patent Document 1).
Among the oxide semiconductors, amorphous oxide semiconductors consisting of indium, gallium, zinc and oxygen (In-Ga—Zn-O, which may hereinafter be referred to as “IGZO”) have preferably been used in particular because of their very high carrier mobility. For example, non-patent literature documents 1 and 2 disclose thin film transistors (TFTs) in which a thin film of an oxide semiconductor having an In:Ga:Zn ratio equal to 1.1:1.1:0.9 (atomic % ratio) was used as a semiconductor layer (active layer).
When an oxide semiconductor is used as a semiconductor layer of a thin film transistor, the oxide semiconductor is required to have a high carrier concentration and a high mobility and excellent TFT switching properties (transistor characteristics or TFT characteristics). Specifically, the oxide semiconductor is required to have (1) a high on-state current (i.e., the maximum drain current when a positive voltage is applied to both a gate electrode and a drain electrode); (2) a low off-state current (i.e., a drain current when a negative voltage is applied to the gate electrode and a positive voltage is applied to the drain voltage, respectively); (3) a low SS value (Subthreshold Swing, i.e., a gate voltage needed to increase the drain current by one digit); (4) a stable threshold value (i.e., a voltage at which the drain current starts to flow when a positive voltage is applied to the drain electrode and either a positive voltage or a negative voltage is applied to the gate voltage, which voltage may also be called as a threshold voltage) showing no change with time (which means that the threshold voltage is uniform in the substrate surface); and (5) a high mobility.
Furthermore, TFTs using an oxide semiconductor layer such as IGZO are required to have excellent resistance to stress such as voltage application and light irradiation (stress stability). It is pointed out that, for example, when a voltage is continuously applied to the gate electrode or when light in a blue emitting band in which light absorption arises is continuously irradiated, electric charges are trapped on the boundary between the gate insulator film and the semiconductor layer of a thin film transistor, which induces a large shift of the threshold voltage toward negative side due to the change of electric charges within the semiconductor layer, resulting in a variation of switching characteristics. When a thin film transistor is used, such variation of the switching characteristics due to the stress by the voltage application and the light irradiation causes deterioration of reliability in a display devices itself.
Similarly in an organic EL display panel, the semiconductor layer is irradiated by light leaked out from a light emission layer, causing problems as a variation and a deviation of the threshold voltage in the TFT.
Such a shift of threshold voltage of the TFT particularly deteriorates the reliability of display devices such as a liquid crystal display and an organic EL display. Therefore, an improvement in the stress stability (a small variation before and after the stress tests) is eagerly desired.
Patent Document 2 is named as an example which improved electrical properties of TFT. The document discloses a technology to lower the hydrogen concentration to smaller than 6×1020 atoms/cm3 in an insulating film, including a gate insulator film, which is in direct contact to an oxide semiconductor layer of a channel region and to suppress diffusion of hydrogen into the oxide semiconductor layer. Diffusion of hydrogen induces excess carrier concentration in the oxide semiconductor layer and negative shift of the threshold voltage, turning the transistor normally-on state in which the drain current flows even without putting the gate bias (Vg=0 V), which makes the transistor faulty. The Patent Document 2 thereby describes that diffusion of hydrogen into the oxide semiconductor layer was suppressed by employing a hydrogen-reduced oxide insulating film for the insulating film which is in direct contact to the oxide semiconductor layer. The document also explains that the electrical properties of a transistor are improved because oxygen is provided from the oxide insulating film to oxygen related defects in the oxide semiconductor layer. Furthermore, according to the Patent Document 2, it is necessary to decrease the hydrogen concentration to smaller than 6×1020 atoms/cm3 in the insulating film in order to secure the effect. It is also stated vital to select and use hydrogen-free gas as the source gas in the process of plasma CVD of the hydrogen-reduced insulating film. In the Patent Document 2, SiF4 is employed for the source gas instead of generally-used SiH4. However, no attention is paid to improving stress stability, particularly decreasing the threshold voltage shift by light and electrical biasing stresses.